A method of the type mentioned in the introduction for producing the polycrystalline ceramic film on the substrate surface of a substrate and a capacitor structure with the ceramic film are known for example from WO 2004/067797 A1. The capacitor structure has a lower electrode layer arranged on the substrate, an upper electrode layer and a polycrystalline piezoelectric ceramic layer arranged between the electrode layers. The capacitor structure (thin layer capacitor) forms a so-called piezoacoustic thin film resonator (Film Bulk Acoustic Resonator, FBAR). The crystalline piezoelectric ceramic layer is formed by the polycrystalline ceramic film. The ceramic film consists of zinc oxide (ZnO). The electrode layers are made of platinum for example. The electrode layers and the piezoelectric layer are arranged adjacent to one another in such a manner that electrical activation of the electrode layers with an electric alternating field results due to the piezo-effect in (acoustic) oscillation of the ceramic film and therefore oscillation of the resonator at a specific resonant frequency. The resonant frequency of the oscillation of the resonator is a function of the layer thicknesses of the layers of the capacitor structure. The mode of oscillation (thickness longitudinal oscillation or thickness shear oscillation) stimulated is a function of a crystal structure of the zinc oxide single crystals and a relative alignment of the zinc oxide single crystals in relation to the applied electric alternating field.
A vapor deposition method is used to generate the layers of the capacitor structure on a substrate, for example a silicon substrate. The lower electrode layer, of polycrystalline platinum for example, is first deposited on the silicon substrate. Zinc oxide is deposited on the lower platinum electrode layer. Without additional measures zinc oxide single crystals grow with a (002) orientation. This means that the polar c-axis of the zinc oxide is oriented perpendicular to the substrate surface or electrode surface. The resulting resonator can therefore be optimally stimulated to thickness longitudinal oscillations.
The known resonator is used to detect a substance in a fluid. To this end the fluid is conducted past a surface segment of the resonator, with the substance to be detected being absorbed on the surface segment. Absorption results in a change in the mass of the resonator and therefore a change in the resonant frequency of the resonator.
If a fluid in the form of a liquid is to be investigated and if the resonant frequency of the resonator is to be determined as the fluid is conducted past, it is particularly advantageous to be able to stimulate the known resonator to thickness shear oscillations. Thickness shear oscillations are virtually not dampened by the fluid, resulting in a relatively high resonator quality and therefore a relatively high level of detectability for the substance in the fluid, compared with thickness longitudinal oscillations. To this end the zinc oxide single crystals have to grow at an angle. The so-called IBAD (Ion Beam Assisted Deposition) method is used according to WO 2004/067797 A1 to adjust the angled preferred direction. While the zinc oxide ceramic film is being deposited, an ion beam, for example a beam of Ar+ ions is directed obliquely onto the substrate surface. The angled growth of the zinc oxide single crystals is achieved with the aid of the ion beam. However the IBAD method is very complex. As an alternative to the IBAD method an angled preferred direction can be achieved by epitactic growth on a suitable single-crystal substrate. This is only possible in a very limited frame. Therefore the crystal structures of the substrate and the ceramic film for example have to be tailored so that epitactic growth is possible. It is also possible to arrange the substrate, on which the ceramic film is to be deposited, obliquely in the particle stream. However this method is only suitable for a relatively small substrate. Use of this method is likewise only possible to a restricted degree.